The present invention relates to a class of substituted triazolo-pyridazine derivatives and to their use in therapy. More particularly, this invention is concerned with substituted 1,2,4-triazolo[4,3-b]pyridazine derivatives which are ligands for GABAA receptors and are therefore useful in the therapy of deleterious mental states.
Receptors for the major inhibitory neurotransmitter, gammaaminobutyric acid (GABA), are divided into two main classes: (1) GABAA receptors, which are members of the ligand-gated ion channel superfamily; and (2) GABAB receptors, which may be members of the G-protein linked receptor superfamily. Since the first cDNAs encoding individual GABAA receptor subunits were cloned the number of known members of the mammalian family has grown to thirteen (six xcex1 subunits, three xcex2 subunits, three xcex3 subunits and one xcex4 subunit). It may be that further subunits remain to be discovered; however, none has been reported since 1993.
Although knowledge of the diversity of the GABAA receptor gene family represents a huge step forward in our understanding of this ligand-gated ion channel, insight into the extent of subtype diversity is still at an early stage. It has been indicated that an xcex1 subunit, a xcex2 subunit and a xcex3 subunit constitute the minimum requirement for forming a fully functional GABAA receptor expressed by transiently transfecting cDNAs into cells. As indicated above, a xcex4 subunit also exists, but is present only to a minor extent in GABAA receptor populations.
Studies of receptor size and visualisation by electron microscopy conclude that, like other members of the ligand-gated ion channel family, the native GABAA receptor exists in pentameric form. The selection of at least one xcex1, one xcex2 and one xcex3 subunit from a repertoire of thirteen allows for the possible existence of more than 10,000 pentameric subunit combinations. Moreover, this calculation overlooks the additional permutations that would be possible if the arrangement of subunits around the ion channel had no constraints (i.e. there could be 120 possible variants for a receptor composed of five different subunits).
Receptor subtype assemblies which do exist include, amongst many others, xcex11xcex22xcex32, xcex12xcex22/3xcex32, xcex13xcex2xcex32/3, xcex12xcex2xcex31, xcex15xcex23xcex32/3, xcex16xcex2xcex32, xcex16xcex2xcex4 and xcex14xcex2xcex4. Subtype assemblies containing an xcex11 subunit are present in most areas of the brain and are thought to account for over 40% of GABAA receptors in the rat. Subtype assemblies containing a2 and a3 subunits respectively are thought to account for about 25% and 17% of GABAA receptors in the rat. Subtype assemblies containing an xcex15 subunit are expressed predominantly in the hippocampus and cortex and are thought to represent about 4% of GABAA receptors in the rat.
A characteristic property of all known GABAA receptors is the presence of a number of modulatory sites, one of which is the benzodiazepine (BZ) binding site. The BZ binding site is the most explored of the GABAA receptor modulatory sites, and is the site through which anxiolytic drugs such as diazepam and temazepam exert their effect. Before the cloning of the GABAA receptor gene family, the benzodiazepine binding site was historically subdivided into two subtypes, BZ1 and BZ2, on the basis of radioligand binding studies. The BZ1 subtype has been shown to be pharmacologically equivalent to a GABAA receptor comprising the xcex11 subunit in combination with a subunit and xcex32. This is the most abundant GABAA receptor subtype, and is believed to represent almost half of all GABAA receptors in the brain.
Two other major populations are the xcex12xcex2xcex32 and xcex13xcex2xcex32/3 subtypes. Together these constitute approximately a further 35% of the total GABAA receptor repertoire. Pharmacologically this combination appears to be equivalent to the BZ2 subtype as defined previously by radioligand binding, although the BZ2 subtype may also include certain xcex15-containing subtype assemblies. The physiological role of these subtypes has hitherto been unclear because no sufficiently selective agonists or antagonists were known.
It is now believed that agents acting as BZ agonists at xcex11xcex2xcex32, xcex12xcex2xcex32 or xcex13xcex2xcex32 subunits will possess desirable anxiolytic properties. Compounds which are modulators of the benzodiazepine binding site of the GABAA receptor by acting as BZ agonists are referred to hereinafter as xe2x80x9cGABAA receptor agonistsxe2x80x9d. The al-selective GABAA receptor agonists alpidem and zolpidem are clinically prescribed as hypnotic agents, suggesting that at least some of the sedation associated with known anxiolytic drugs which act at the BZ1 binding site is mediated through GABAA receptors containing the xcex11 subunit. Accordingly, it is considered that GABAA receptor agonists which bind more effectively to the xcex12 and/or xcex13 subunit than to xcex11 will be effective in the treatment of anxiety with a reduced propensity to cause sedation. Also, agents which are antagonists or inverse agonists at xcex11 might be employed to reverse sedation or hypnosis caused by xcex11 agonists.
The compounds of the present invention, being selective ligands for GABAA receptors, are therefore of use in the treatment and/or prevention of a variety of disorders of the central nervous system. Such disorders include anxiety disorders, such as panic disorder with or without agoraphobia, agoraphobia without history of panic disorder, animal and other phobias including social phobias, obsessive-compulsive disorder, stress disorders including post-traumatic and acute stress disorder, and generalized or substance-induced anxiety disorder; neuroses; convulsions; migraine; and depressive or bipolar disorders, for example single-episode or recurrent major depressive disorder, dysthymic disorder, bipolar I and bipolar II manic disorders, and cyclothymic disorder.
In DE-A-2741763, and in U.S. Pat. Nos. 4,260,755, 4,260,756 and 4,654,343, are described various classes of 1,2,4-triazolo[4,3-b]pyridazine derivatives which are alleged to be useful as anxiolytic agents. The compounds described in DE-A-2741763 and in U.S. Pat. Nos. 4,260,755 and 4,654,343 possess a phenyl substituent at the 6-position of the triazolo-pyridazine ring system. The compounds described in U.S. Pat. No. 4,260,756, meanwhile, possess a heteroaryl moiety at the 6- or 8-position. In none of these publications, however, is there any disclosure or suggestion of 1,2,4-triazolo[4,3-b]pyridazine derivatives wherein the substituent at the 6-position is attached through a directly linked oxygen atom.
EP-A-0085840 and EP-A-0134946 describe related series of 1,2,4-triazolo[3,4-a]phthalazine derivatives which are stated to possess antianxiety activity. However, there is no disclosure nor any suggestion in either of these publications of replacing the benzo moiety of the triazolo-phthalazine ring system with any other functionality.
The present invention provides a class of triazolo-pyridazine derivatives which possess desirable binding properties at various GABAA receptor subtypes. The compounds in accordance with the present invention have good affinity as ligands for the xcex12 and/or xcex13 subunit of the human GABAA receptor. The compounds of this invention may display more effective binding to the xcex12 and/or xcex13 subunit than to the xcex11 subunit. Desirably, the compounds of the invention will exhibit functional selectivity in terms of a selective efficacy for the xcex12 and/or xcex13 subunit relative to the xcex11 subunit.
The compounds of the present invention are GABAA receptor subtype ligands having a binding affinity (Ki) for the xcex12 and/or xcex13 subunit, as measured in the assay described hereinbelow, of 100 nM or less, typically of 50 nM or less, and ideally of 10 nM or less. The compounds in accordance with this invention may possess at least a 2-fold, suitably at least a 5-fold, and advantageously at least a 10-fold, selective affinity for the xcex12 and/or xcex13 subunit relative to the xcex11 subunit. However, compounds which are unselective in terms of their binding affinity for the xcex12 and/or xcex13 subunit relative to the xcex11 subunit are also encompassed within the scope of the present invention; such compounds will desirably exhibit functional selectivity in terms of a selective efficacy for the xcex12 and/or xcex13 subunit relative to the xcex11 subunit.
A number of dementing illnesses such as Alzheimer""s disease are characterised by a progressive deterioration in cognition in the sufferer. It would clearly be desirable to enhance cognition in subjects desirous of such treatment, for example for subjects suffering from a dementing illness.
It has been reported by McNamara and Skelton in Psychobiology, 21:101-108, that the benzodiazepine receptor inverse agonist xcex2-CCM enhanced spatial learning in the Morris watermaze. However, xcex2-CCM and other conventional benzodiazepine receptor inverse agonists are proconvulsant or convulsant which makes it clear that they cannot be used as cognition enhancing agents in humans.
However, we have now discovered that it is possible to obtain medicaments which have cognition enhancing effects which may be employed with less risk of proconvulsant effects previously described with benzodiazepine receptor partial or full inverse agonists.
It has now been discovered that use of an xcex15 receptor partial or full inverse agonist which is relatively free of activity at xcex11 and/or xcex12 and/or xcex13 receptor binding sites can be used to provide a medicament which is useful for enhancing cognition but in which proconvulsant activity is reduced or eliminated. Inverse agonists at xcex15 which are not free of activity at xcex11 and/or xcex12 and/or xcex13 but which are functionally selective for xcex15 can also be used. Inverse agonists which are both selective for xcex15 and are relatively free of activity at al, xcex12 and xcex13 receptor binding sites are preferred.
European Patent Applications 0085840 and 0134946 describe related series of 1,2,4-triazolo[3,4-a]phthalazine derivatives which are stated to possess antianxiety activity. However, there is no disclosure nor any suggestion in either of these publications of the compounds of the present invention, nor that the compounds disclosed in the Applications have any cognition enhancing properties.
The present invention provides a compound of formula I, or a salt or prodrug thereof: 
wherein
Z represents C1-6 alkyl, C3-7 cycloalkyl, C4-7 cycloalkenyl, aryl, C3-7 heterocycloalkyl, heteroaryl or di(C1-6)alkylamino, any of which groups may be optionally substituted;
R1 represents an optionally substituted five-membered heteroaromatic ring selected from oxazole, thiazole, isoxazole, isothiazole, imidazole, pyrazole, oxadiazole, thiadiazole, triazole and tetrazole; or
R1 represents an optionally substituted six-membered heteroaromatic ring selected from pyrazine, pyrimidine and pyridazine; and
R2 represents cyano(C1-6)alkyl, hydroxy(C1-6)alkyl, C3-7 cycloalkyl(C1-6)alkyl, propargyl, C3-7 heterocycloalkylcarbonyl(C1-6)alkyl, aryl(C1-6)alkyl or heteroaryl(C1-6)alkyl, any of which groups may be optionally substituted.
The groups Z, R1 and R2 may be unsubstituted, or substituted by one or more, suitably by one or two, substituents. In general, the groups Z, R1 and R2 will be unsubstituted or monosubstituted. Examples of optional substituents on the groups Z, R1 and R2 include C1-6 alkyl, aryl(C1-6)alkyl, pyridyl(C1-6)alkyl, halogen, halo(C1-6)alkyl, cyano, cyano(C1-6)alkyl, hydroxy, hydroxymethyl, C1-6 alkoxy, C3-7 cycloalkyl(C1-6)alkoxy, C3-7 cycloalkoxy, amino(C1-6)alkyl, di(C1-6)alkylamino(C1-6)alkyl, di(C1-6)alkylaminocarbonyl(C1-6)alkyl, N-(C1-6)alkylpiperidinyl, pyrrolidinyl(C1-6)alkyl, piperazinyl(C1-6)alkyl, morpholinyl(C1-6)alkyl, di(C1-6)alkylmorpholinyl(C1-6)alkyl and imidazolyl(C1-6)alkyl.
As used herein, the expression xe2x80x9cC1-6 alkylxe2x80x9d includes methyl and ethyl groups, and straight-chained or branched propyl, butyl, pentyl and hexyl groups. Particular alkyl groups are methyl, ethyl, n-propyl, isopropyl, tert-butyl and 1,1-dimethylpropyl. Derived expressions such as xe2x80x9cC1-6 alkoxyxe2x80x9d are to be construed accordingly.
Typical C3-7 cycloalkyl groups include cyclopropyl, cyclobutyl, cyclopentyl and cyclohexyl.
The expression xe2x80x9cC3-7 cycloalkyl(C1-6)alkylxe2x80x9d as used herein includes cyclopropylmethyl, cyclobutylmethyl, cyclopentylmethyl and cyclohexylmethyl.
Typical C4-7 cycloalkenyl groups include cyclobutenyl, cyclopentenyl and cyclohexenyl.
Typical aryl groups include phenyl and naphthyl, preferably phenyl.
The expression xe2x80x9caryl(C1-6)alkylxe2x80x9d as used herein includes benzyl, phenylethyl, phenylpropyl and naphthylmethyl.
Suitable heterocycloalkyl groups include azetidinyl, pyrrolidinyl, piperidinyl, piperazinyl, morpholinyl and thiomorpholinyl groups.
Suitable heteroaryl groups include pyridinyl, quinolinyl, isoquinolinyl, pyridazinyl, pyrimidinyl, pyrazinyl, quinoxalinyl, furyl, benzofuryl, dibenzofuryl, thienyl, benzthienyl, pyrrolyl, indolyl, pyrazolyl, indazolyl, oxazolyl, isoxazolyl, thiazolyl, isothiazolyl, imidazolyl, benzimidazolyl, oxadiazolyl, thiadiazolyl, triazolyl and tetrazolyl groups.
The expression xe2x80x9cheteroaryl(C1-6)alkylxe2x80x9d as used herein includes furylmethyl, furylethyl, thienylmethyl, thienylethyl, pyrazolylmethyl, oxazolylmethyl, oxazolylethyl, isoxazolylmethyl, thiazolylmethyl, thiazolylethyl, imidazolylmethyl, imidazolylethyl, benzimidazolylmethyl, oxadiazolylmethyl, oxadiazolylethyl, thiadiazolylmethyl, thiadiazolylethyl, triazolylmethyl, triazolylethyl, tetrazolylmethyl, tetrazolylethyl, pyridinylmethyl, pyridinylethyl, pyridazinylmethyl, pyrimidinylmethyl, pyrazinylmethyl, quinolinylmethyl, isoquinolinylmethyl and quinoxalinylmethyl.
The term xe2x80x9chalogenxe2x80x9d as used herein includes fluorine, chlorine, bromine and iodine, especially fluorine or chlorine.
For use in medicine, the salts of the compounds of formula I will be pharmaceutically acceptable salts. Other salts may, however, be useful in the preparation of the compounds according to the invention or of their pharmaceutically acceptable salts. Suitable pharmaceutically acceptable salts of the compounds of this invention include acid addition salts which may, for example, be formed by mixing a solution of the compound according to the invention with a solution of a pharmaceutically acceptable acid such as hydrochloric acid, sulphuric acid, methanesulphonic acid, fumaric acid, maleic acid, succinic acid, acetic acid, benzoic acid, oxalic acid, citric acid, tartaric acid, carbonic acid or phosphoric acid. Furthermore, where the compounds of the invention carry an acidic moiety, suitable pharmaceutically acceptable salts thereof may include alkali metal salts, e.g. sodium or potassium salts; alkaline earth metal salts, e.g. calcium or magnesium salts; and salts formed with suitable organic ligands, e.g. quaternary ammonium salts.
The present invention includes within its scope prodrugs of the compounds of formula I above. In general, such prodrugs will be functional derivatives of the compounds of formula I which are readily convertible in vivo into the required compound of formula I. Conventional procedures for the selection and preparation of suitable prodrug derivatives are described, for example, in Design of Prodrugs, ed. H. Bundgaard, Elsevier, 1985.
Where the compounds according to the invention have at least one asymmetric centre, they may accordingly exist as enantiomers. Where the compounds according to the invention possess two or more asymmetric centres, they may additionally exist as diastereoisomers. It is to be understood that all such isomers and mixtures thereof in any proportion are encompassed within the scope of the present invention.
Examples of suitable values for the substituent Z include methyl, ethyl, isopropyl, tert-butyl, 1,1-dimethylpropyl, methyl-cyclopropyl, cyclobutyl, methyl-cyclobutyl, cyclopentyl, methyl-cyclopentyl, cyclohexyl, cyclobutenyl, phenyl, pyrrolidinyl, methyl-pyrrolidinyl, piperidinyl, morpholinyl, thiomorpholinyl, pyridinyl, furyl, thienyl, chloro-thienyl and diethylamino. A particular value of Z is phenyl.
Suitable values for the moiety R1 include thiazolyl, isoxazolyl and pyrazinyl, any of which groups may be optionally substituted by one or more substituents. Suitably R1 is unsubstituted, or substituted by one or two substituents. In general, R1 will be unsubstituted or monosubstituted. A particular substituent on the moiety R1 is C1-6 alkyl, especially methyl.
Particular values of R1 include methylthiazolyl (especially 4-methyl-thiazol-2-yl), methylisoxazolyl (especially 5-methylisoxazol-3-yl) and pyrazinyl (especially pyrazin-2-yl).
Suitable values for the substituent R2 in the compounds according to the invention include cyanomethyl, hydroxybutyl, cyclohexylmethyl, propargyl, pyrrolidinylcarbonylmethyl, benzyl, pyrazolylmethyl, isoxazolylmethyl, thiazolylmethyl, thiazolylethyl, imidazolylmethyl, benzimidazolylmethyl, oxadiazolylmethyl, triazolylmethyl, tetrazolylmethyl, pyridinylmethyl, pyridazinylmethyl, pyrimidinylmethyl, pyrazinylmethyl, quinolinylmethyl, isoquinolinylmethyl and quinoxalinylmethyl, any of which groups may be optionally substituted by one or more substituents. Typical values of R2 include triazolylmethyl and pyridinylmethyl, either of which groups may be optionally substituted by one or more substituents.
Examples of suitable optional substituents on the group R2 include C1-6 alkyl, aryl(C1-6)alkyl, pyridyl(C1-6)alkyl, halogen, halo(C1-6)alkyl, cyano, cyano(C1-6)alkyl, hydroxymethyl, C1-6 alkoxy, C3-7 cycloalkyl(C1-6)alkoxy, amino(C1-6)alkyl, di(C1-6)alkylamino(C1-6)alkyl, di(C1-6)alkylaminocarbonyl(C1-6)alkyl, N-(C1-6)alkylpiperidinyl, pyrrolidinyl(C1-6)alkyl, piperazinyl(C1-6)alkyl, morpholinyl(C1-6)alkyl and di(C1-6)alkylmorpholinyl(C1-6)alkyl, especially C1-6 alkyl.
Specific illustrations of particular substituents on the group R2 include methyl, ethyl, n-propyl, benzyl, pyridinylmethyl, chloro, chloromethyl, cyano, cyanomethyl, hydroxymethyl, ethoxy, cyclopropylmethoxy, dimethylaminomethyl, aminoethyl, dimethylaminoethyl, dimethylaminocarbonylmethyl, N-methylpiperidinyl, pyrrolidinylethyl, piperazinylethyl, morpholinylmethyl and dimethylmorpholinylmethyl, especially methyl.
Representative values of R2 include cyanomethyl, hydroxybutyl, hydroxymethyl-cyclohexylmethyl, propargyl, dimethylaminomethyl-propargyl, dimethylmorpholinylmethyl-propargyl, pyrrolidinylcarbonylmethyl, cyanobenzyl, hydroxymethyl-benzyl, pyrazolylmethyl, dimethyl-pyrazolylmethyl, methyl-isoxazolylmethyl, thiazolylmethyl, methyl-thiazolylmethyl, ethyl-thiazolylmethyl, methyl-thiazolylethyl, imidazolylmethyl, methyl-imidazolylmethyl, ethyl-imdazolylmethyl, benzyl-imidazolylmethyl, benzimidazolylmethyl, methyl-oxadiazolylmethyl, triazolylmethyl, methyl-triazolylmethyl, propyl-triazolylmethyl, benzyl-triazolylmethyl, pyridinylmethyl-triazolylmethyl, cyanomethyl-triazolylmethyl, dimethylaminomethyl-triazolylmethyl, aminoethyl-triazolylmethyl, dimethylaminoethyl-triazolylmethyl, dimethylaminocarbonylmethyl-triazolylmethyl, N-methylpiperidinyl-triazolylmethyl, pyrrolidinylethyl-triazolylmethyl, piperazinylethyl-triazolylmethyl, morpholinylethyl-triazolylmethyl, methyl-tetrazolylmethyl, pyridinylmethyl, methyl-pyridinylmethyl, dimethyl-pyridinylmethyl, ethoxy-pyridinylmethyl, cyclopropylmethoxy-pyridinylmethyl, pyridazinylmethyl, chloro-pyridazinylmethyl, pyrimidinylmethyl, pyrazinylmethyl, quinolinylmethyl, isoquinolinylmethyl and quinoxalinylmethyl.
Particular values of R2 include triazolylmethyl, methyl-triazolylmethyl and pyridinylmethyl.
A particular sub-class of compounds according to the invention is represented by the compounds of formula IIA, and salts and prodrugs thereof: 
wherein R1 is as defined above;
Z1 represents C1-6 alkyl, C3-7 cycloalkyl or aryl; and
R12 represents optionally substituted heteroaryl.
Examples of optional substituents on the group R12 suitably include C1-6 alkyl, especially methyl.
Particular values of R12 include triazolyl, methyltriazolyl and pyridinyl.
Particular values of Z1 include tert-butyl, cyclobutyl, cyclopentyl and phenyl, especially phenyl.
Specific compounds within the scope of the present invention include:
6-(1-methyl-1H-1,2,4-triazol-3-ylmethyloxy)-7-phenyl-3-(pyrazin-2-yl)-1,2,4-triazolo[4,3-b]pyridazine;
6-(2-methyl-2H-1,2,4-triazol-3-ylmethyloxy)-7-phenyl-3-(pyrazin-2-yl)-1,2,4-triazolo[4,3-b]pyridazine;
7-phenyl-3-(pyrazin-2-yl)-6-(1H-1,2,4-triazol-3-ylmethyloxy)-1,2,4-triazolo[4,3-b]pyridazine;
3-(4-methylthiazol-2-yl)-7-phenyl-6-(pyridin-2-ylmethyloxy)-1,2,4-triazolo[4,3-b]pyridazine;
and salts and prodrugs thereof.
A further particular sub-class of compounds according to the invention is represented by the compounds of formula IIB, and salts and prodrugs thereof: 
wherein:
R1 and R2 are as defined above; and
Z2 is tertiary butyl, cyclobutyl or methylcyclobutyl.
With respect to compounds of formula (IIB), R1 is preferably an optionally substituted five-membered heteroaromatic ring selected from oxazole, thiazole, isoxazole, isothiazole, imidazole, pyrazole, oxadiazole, thiadiazole, triazole and tetrazole wherein the optional substituents are selected from C1-6alkyl, C1-6alkoxy, halogen, halo(C1-6)alkyl, hydroxy, C3-7cycloalkyl and C3-7cycloalkoxy. Preferably the optional substituents are selected from C1-6alkyl, C1-6alkoxy and halogen and more preferably selected from methyl and chloro, especially methyl.
Preferably R1 is unsubstituted or monosubstituted and more preferably monosubstituted.
R1 is most preferably an optionally substituted isoxazole or oxadiazole such as an isoxazol-3-yl or a 1,2,4-oxadiazol-3-yl. R1 is thus preferably a methylisoxazole, isoxazole, chloroisoxazole or methyloxadiazole, particularly a 5-methylisoxazol-3-yl, 5-chloroisoxazol-3-yl, isoxazol-3-yl or 5-methyl-1,2,4-oxadiazol-3-yl group.
With respect to compounds of formula (IIB), R2 is preferably a heteroaryl(C1-6)alkyl. The heteroaryl group is preferably a 5-membered heteroaromatic ring containing 1, 2, 3 or 4 heteroatoms independently chosen from oxygen, nitrogen and sulphur, at most one of the heteroatoms being oxygen or sulphur, or a 6-membered heteroaromatic ring containing 1, 2, or 3 nitrogen atoms, the heteroaromatic ring being optionally substituted by one, two or three groups independently chosen from C1-6alkyl, C1-6alkoxy, halogen, phenylC1-6alkyl, amino and hydroxy.
When R2 is pyridyl it may be in the form of the N-oxide.
R2 is preferably a pyridine, triazole or imidazole.
When R2 is substituted it is preferably monosubstituted.
Preferred substituents on R2 are C1-6alkyl groups such as methyl. Thus monosubstitution by methyl (especially when R2 is a five-membered heteroaromatic ring) is favoured. When R1 is a six-membered heteroaromatic ring it is preferably unsubstituted.
Particular examples of R2 are pyrid-2-yl, 3-methyl-1,2,3-triazol-5-yl, 1-benzylimidazol-2-yl, 2-methyl-1,2,4-triazol-3-yl, 1-methyl-1,2,4-triazol-3-yl and 3-methyl-1,2,3-triazol-4-yl.
Other examples are 1-methyl-1,2,3-triazol-4-yl and 1-methyl-1,2,3-triazol-5-yl.
Thus a preferred subclass of compounds of formula (IIB) is wherein:
Z2 is as defined above;
R1 is isoxazole or oxadiazole optionally substituted by chlorine or methyl; and
R2 is pyridine; or imidazole or a triazole optionally substituted by methyl or benzyl and preferably substituted by methyl.
When Z2 is a methylcyclobutyl group, 1-methylcyclobutyl is particularly preferred. In particular, Z2 is preferably a tertiary butyl group.
Specific compounds within the scope of this subclass include:
7-t-butyl-3-(5-methylisoxazol-3-yl)-6-(2-pyridylmethyloxy)-1,2,4-triazolo[4,3-b]pyridazine;
7-t-butyl-3-(5-methylisoxazol-3-yl)-6-(1-methyl-1,2,4-triazol-3-yl)methyloxy-1,2,4-triazolo[4,3-b]pyridazine;
7-t-butyl-3-(5-methylisoxazol-3-yl)-6-(1-methyl-1,2,3-triazol-4-yl)methyloxy-1,2,4-triazolo[4,3-b]pyridazine;
and salts and prodrugs thereof.
Also provided by the present invention is a method for the treatment and/or prevention of anxiety which comprises administering to a patient in need of such treatment an effective amount of a compound of formula I as defined above or a pharmaceutically acceptable salt thereof or a prodrug thereof.
Further provided by the present invention is a method for the treatment and/or prevention of convulsions (e.g. in a patient suffering from epilepsy or a related disorder) which comprises administering to a patient in need of such treatment an effective amount of a compound of formula I as defined above or a pharmaceutically acceptable salt thereof or a prodrug thereof.
The binding affinity (Ki) of the compounds according to the present invention for the xcex13 subunit of the human GABAA receptor is conveniently as measured in the assay described hereinbelow. The xcex13 subunit binding affinity (Ki) of compounds according to the invention is ideally 10 nM or less, preferably 2 nM or less, and more preferably 1 nM or less.
The compounds according to the present invention will ideally elicit at least a 40%, preferably at least a 50%, and more preferably at least a 60%, potentiation of the GABA EC20 response in stably transfected recombinant cell lines expressing the xcex13 subunit of the human GABAA receptor. Moreover, the compounds of the invention will ideally elicit at most a 30%, preferably at most a 20%, and more preferably at most a 10%, potentiation of the GABA EC20 response in stably transfected recombinant cell lines expressing the xcex11 subunit of the human GABAA receptor.
The potentiation of the GABA EC20 response in stably transfected cell lines expressing the xcex13 and xcex11 subunits of the human GABAA receptor can conveniently be measured by procedures analogous to the protocol described in Wafford et al., Mol. Pharmacol., 1996, 50, 670-678. The procedure will suitably be carried out utilising cultures of stably transfected eukaryotic cells, typically of stably transfected mouse Ltkxe2x88x92 fibroblast cells.
The compounds according to the present invention exhibit anxiolytic activity, as demonstrated by a positive response in the elevated plus maze and conditioned suppression of drinking tests (cf. Dawson et al., Psychopharmacology, 1995, 121, 109-117). Moreover, the compounds of the invention are substantially non-sedating, as confirmed by an appropriate result obtained from the response sensitivity (chain-pulling) test (cf. Bayley et al., J. Psychopharmacol., 1996, 10, 206-213).
The compounds according to the present invention also exhibit anticonvulsant activity. This is demonstrated by their ability to block pentylenetetrazole-induced seizures in rats and mice, following a protocol analogous to that described by Bristow et al. in J. Pharmacol. Exp. Ther., 1996, 279,492-501.
In order to elicit their behavioural effects, the compounds of the invention will be brain-penetrant; in other words, these compounds will be capable of crossing the so-called xe2x80x9cblood-brain barrierxe2x80x9d. Preferably, the compounds of the invention will be capable of exerting their beneficial therapeutic action following administration by the oral route.
The invention also provides pharmaceutical compositions comprising one or more compounds of this invention in association with a pharmaceutically acceptable carrier. Preferably these compositions are in unit dosage forms such as tablets, pills, capsules, powders, granules, sterile parenteral solutions or suspensions, metered aerosol or liquid sprays, drops, ampoules, auto-injector devices or suppositories; for oral, parenteral, intranasal, sublingual or rectal administration, or for administration by inhalation or insufflation. For preparing solid compositions such as tablets, the principal active ingredient is mixed with a pharmaceutical carrier, e.g. conventional tableting ingredients such as corn starch, lactose, sucrose, sorbitol, talc, stearic acid, magnesium stearate, dicalcium phosphate or gums, and other pharmaceutical diluents, e.g. water, to form a solid preformulation composition containing a homogeneous mixture of a compound of the present invention, or a pharmaceutically acceptable salt thereof. When referring to these preformulation compositions as homogeneous, it is meant that the active ingredient is dispersed evenly throughout the composition so that the composition may be readily subdivided into equally effective unit dosage forms such as tablets, pills and capsules. This solid preformulation composition is then subdivided into unit dosage forms of the type described above containing from 0.1 to about 500 mg of the active ingredient of the present invention. Typical unit dosage forms contain from 1 to 100 mg, for example 1, 2, 5, 10, 25, 50 or 100 mg, of the active ingredient. The tablets or pills of the novel composition can be coated or otherwise compounded to provide a dosage form affording the advantage of prolonged action. For example, the tablet or pill can comprise an inner dosage and an outer dosage component, the latter being in the form of an envelope over the former. The two components can be separated by an enteric layer which serves to resist disintegration in the stomach and permits the inner component to pass intact into the duodenum or to be delayed in release. A variety of materials can be used for such enteric layers or coatings, such materials including a number of polymeric acids and mixtures of polymeric acids with such materials as shellac, cetyl alcohol and cellulose acetate.
The liquid forms in which the novel compositions of the present invention may be incorporated for administration orally or by injection include aqueous solutions, suitably flavoured syrups, aqueous or oil suspensions, and flavoured emulsions with edible oils such as cottonseed oil, sesame oil, coconut oil or peanut oil, as well as elixirs and similar pharmaceutical vehicles. Suitable dispersing or suspending agents for aqueous suspensions include synthetic and natural gums such as tragacanth, acacia, alginate, dextran, sodium carboxymethylcellulose, methylcellulose, polyvinyl-pyrrolidone or gelatin.
In the treatment of anxiety, a suitable dosage level is about 0.01 to 250 mg/kg per day, preferably about 0.05 to 100 mg/kg per day, and especially about 0.05 to 5 mg/kg per day. The compounds may be administered on a regimen of 1 to 4 times per day.
The compounds of the present invention have a good binding affinity (Ki) for the xcex15 subunit. In a preferred embodiment the compounds of the invention are binding selective for the xcex15 subunit relative to the xcex11, xcex12 and xcex13 subunits. In another preferred embodiment the compounds are functionally selective for the xcex15 subunit as partial or full inverse agonists whilst substantially being antagonists at the xcex11, xcex12 and xcex13 subunits. In particular the compounds of formula IIB are generally inverse agonists of the xcex15 subunit.
Cognition enhancement can be shown by testing the compounds in the Morris watermaze as reported by McNamara and Skelton, Psychobiology, 21:101-108. The functional efficacy at the various receptor subtypes can be calculated using the method disclosed in WO-A-9625948.
The present invention also provides a compound of the invention for use in a method of treatment of the human body. Preferably the treatment is for a condition associated with GABAA receptors comprising the xcex15 subunit and/or for the enhancement of cognition. Preferably the condition is a neurological deficit with an associated cognitive disorder such as a dementing illness such as Alzheimer""s disease. Other conditions to be treated include cognition deficits due to traumatic injury, stroke, Parkinson""s disease, Downs syndrome, age related memory deficits, attention deficit disorder and the like.
Thus, for example, the compounds of the present invention can be used in a variety of disorders of the central nervous system. Such disorders include delirium, dementia and amnestic and other cognitive disorders. Examples of delirium are delirium due to substance intoxication or substance withdrawal, delirium due to multiple etiologies and delirium NOS (not otherwise specified). Examples of dementia are: dementia of the Alzheimer""s type with early onset which can be uncomplicated or with delirium, delusions or depressed mood; dementia of the Alzheimer""s type, with late onset, which can be uncomplicated or with delirium, delusions or depressed mood; vascular dementia which can be uncomplicated or with delirium, delusions or depressed mood; dementia due to HIV disease; dementia due to head trauma; dementia due to Parkinson""s disease; dementia due to Huntington""s disease; dementia due to Pick""s disease; dementia due to Creutzfeld-Jakob disease; dementia which is substance-induced persisting or due to multiple etiologies; and dementia NOS. Examples of amnestic disorders are amnestic disorder due to a particular medical condition or which is substance-induced persisting or which is amnestic disorder NOS.
Those compounds which are not inverse agonists at the xcex15 subtype may be used as alcohol antagonists or to treat obesity.
The present invention further provides the use of a compound of the present invention in the manufacture of a medicament for the enhancement of cognition, preferably in a human suffering from a dementing illness such as Alzheimer""s disease.
Also disclosed is a method of treatment of a subject suffering from a cognition deficit, such as that resulting from a dementing illness such as Alzheimer""s disease, which comprises administering to that subject an effective amount of a compound according to the present invention.
For the enhancement of cognition, a suitable dosage level is about 0.01 to 250 mg/kg per day, preferably about 0.01 to 100 mg/kg per day, and especially about 0.01 to 5 mg/kg of body weight per day. The compounds may be administered on a regimen of 1 to 4 times per day. In some cases, however, dosage outside these limits may be used.
It is preferred that the compounds of the present invention are ground, for example using a pestle and mortar or industrial equivalent thereto, to a particle size of between 1 and 10 xcexcM, and preferably less than 5 xcexcM, before formulation. The compounds may be micronised or sonicised by methods known in the art or nanonised, for example by methods disclosed in US-A-5145684.
The compounds in accordance with the present invention may be prepared by a process which comprises reacting a compound of formula III with a compound of formula IV: 
wherein Z, R1 and R2 are as defined above; and L1 represents a suitable leaving group.
The leaving group L1 is typically a halogen atom, especially chloro.
The reaction between compounds III and IV is conveniently effected by stirring the reactants in a suitable solvent, typically N,N-dimethyl-formamide, in the presence of a strong base such as sodium hydride or lithium bis(trimethylsilyl)amide.
The intermediates of formula III above may be prepared by reacting a compound of formula V with a substantially equimolar amount of a hydrazine derivative of formula VI: 
wherein Z, R1 and L1 are as defined above, and L2 represents a suitable leaving group; followed, if necessary, by separation of the resulting mixture of isomers by conventional means.
The leaving group L2 is typically a halogen atom, especially chloro. In the intermediates of formula V, the leaving groups L1 and L2 may be the same or different, but are suitably the same, preferably both chloro.
The reaction between compounds V and VI is conveniently effected by heating the reactants in the presence of a base such as triethylamine, typically at reflux in an inert solvent such as xylene or 1,4-dioxane.
The reaction between compounds V and VI will, as indicated above, usually give rise to a mixture of isomeric products depending upon whether the hydrazine derivative VI displaces the leaving group L1 or L2. Thus, in addition to the required product of formula III, the isomeric compound wherein the Z moiety is in the adjacent position will usually be obtained to some extent. For this reason it will generally be necessary to separate the resulting mixture of isomers by conventional methods such as chromatography.
In another procedure, the compounds according to the invention may be prepared by a process which comprises reacting a compound of formula VII with a compound of formula VIII: 
wherein Z, R1 and R2 are as defined above; and L3 represents a suitable leaving group.
The leaving group L3 is suitably a halogen atom, typically chloro or bromo.
The reaction between compounds VII and VIII is conveniently effected by stirring the reactants in a suitable solvent, typically N,N-dimethylformamide, in the presence of a strong base such as sodium hydride.
The intermediates of formula VII above may conveniently be prepared by reacting a compound of formula III as defined above with an alkali metal hydroxide, e.g. sodium hydroxide. The reaction is conveniently effected in an inert solvent such as aqueous 1,4-dioxane, ideally at the reflux temperature of the solvent.
In a further procedure, the compounds according to the invention may be prepared by a process which comprises reacting a compound of formula Zxe2x80x94CO2H with a compound of formula IX: 
wherein Z, R1 and R2 are as defined above; in the presence of silver nitrate and ammonium persulphate.
The reaction is conveniently carried out under acidic conditions in a suitable solvent, for example using sulphuric acid in aqueous acetonitrile, typically at an elevated temperature, e.g. the reflux temperature of the solvent.
The intermediates of formula IX correspond to the compounds of formula I as defined above wherein Z is hydrogen, and they may therefore be prepared by methods analogous to those described above for preparing the corresponding compounds of formula I.
The intermediates of formula V above may suitably be prepared by reacting a compound of formula Zxe2x80x94CO2H with a compound of formula X: 
wherein L1 and L2 are as defined above; using procedures analogous to that described above for the reaction between the compound of formula Zxe2x80x94CO2H and compound IX.
In a still further procedure, the compounds of formula I as defined above may be prepared by a process which comprises reacting a compound of formula XI with a compound of formula XII: 
wherein Z, R1 and R2 are as defined above, Alk represents a C1-6 alkyl group, typically n-butyl, and L4 represents a suitable leaving group; in the presence of a transition metal catalyst.
The leaving group L4 is suitably a halogen atom, e.g. bromo.
A suitable transition metal catalyst of use in the reaction between compounds XI and XII comprises dichlorobis(triphenylphosphine)palladium(II).
The reaction between compounds XI and XII is conveniently effected in an inert solvent such as N,N-dimethylformamide, typically at an elevated temperature.
The intermediates of formula XI may be prepared by reacting a compound of formula IV as defined above with a compound of formula XIII: 
wherein Z, L1 and L4 are as defined above; under conditions analogous to those described above for the reaction between compounds III and IV.
Where they are not commercially available, the starting materials of formula IV, VI, VIII, X, XII and XIII may be prepared by methods analogous to those described in the accompanying Examples, or by standard methods well known from the art.
According to yet further alternative methodology, intermediates of formula (III) may be prepared by reacting a compound of formula (XIV) with a compound of formula (XV): 
wherein Z, L1 and R1 are as defined above, and W represents a suitable leaving group such as C1-6alkoxy, chlorine or hydroxy.
The reaction is advantageously conducted in an inert organic solvent, generally in the presence of an organic nitrogen base and preferably under an inert atmosphere such as nitrogen. Suitable solvents include xylene, dioxane, tetrahydrofuran and lower aliphatic halogenated and aromatic hydrocarbons. Suitable organic nitrogen bases that may be employed include trialkylamines and pyridine. The reaction is generally conducted at a temperature range of from xe2x88x9220xc2x0 C. to the reflux temperature of the reaction mixture, for a period of time that depends on the reactants employed and the temperature at which the reaction is carried out. The compound of formula (XV) may be activated by reacting with a compound such as bis-(2-oxo-3-oxazolidinyl)phosphinic chloride or 1,1xe2x80x2-dicarbonyldiimidazole before reaction with the hydrazine.
Alternatively, intermediates of formula (III) in which R1 is an unsubstituted isoxazol-3-yl group or an isoxazol-3-yl group substituted by methyl, chloro or ethoxy, may be prepared from a chloroxime of formula (XVI): 
by reaction with an appropriate unsaturated compound to effect a cyclisation reaction. Generally, the reaction is effected in the presence of a base such as triethylamine in a suitable solvent such as dichloromethane.
Thus, for instance, where R1 is a 5-methylisoxazol-3-yl group, a compound of formula (XVI) is reacted with 2-acetoxypropene; or 2-(tert-butyldimethylsilyloxy)propene; or propyne. Where R1 is an unsubstituted isoxazol-3-yl group, vinyl acetate in xylene at reflux may be used. Where R1 is a 5-chloroisoxazol-3-yl group, vinylidine chloride may be used. Where R1 is a 5-ethoxyisoxazol-3-yl group, ethyl ethynyl ether may be used. Other substituents on the isoxazolyl group may be incorporated in an analogous manner.
Compounds of formula (XVI) are conveniently prepared from the corresponding oxime by a chlorination reaction using, for example, N-chlorosuccinimide in a solvent such as DMF.
The oxime may be prepared in a two-step reaction from a compound of formula (XIV) by reaction with dichloroacetic acid in a solvent such a toluene at reflux, followed by reaction with hydroxylamine hydrochloride in the presence of an acid such as trifluoroacetic acid at reflux.
Compounds of formula (XIV) may be prepared from a compound of formula (V) by reaction with hydrazine monohydrate in a solvent such as ethanol at reflux.
In an alternative procedure, intermediates of formula (III) in which R1 is 1,2,4-oxadiazolyl group may be prepared in an analogous manner to that previously described for the isoxazolyl derivatives, using a compound of formula (XVII): 
Thus, for example, to prepare a compound of formula (III) wherein R1 is a 5-methyl-1,2,4-oxadiazol-3-yl group, a compound of formula (XVII) may be reacted with acetyl chloride in pyridine or by heating with acetic acid at reflux. Other substituents on the oxadiazolyl group may be incorporated using an appropriate acyl chloride or an appropriate acid, or by other methods well known to a person skilled in the art.
Compounds of formula (XVII) may be prepared by reaction of a compound of formula (XVI) with ammonium hydroxide in a solvent such as ethanol.
Where they are not commercially available, compounds of formula (XV) may be prepared by methods analogous to those described in the accompanying Examples, or by standard methods well known in the art.
It will be understood that any compound of formula I initially obtained from any of the above processes may, where appropriate, subsequently be elaborated into a further compound of formula I by techniques known from the art. For example, a compound of formula I initially obtained wherein R2 is unsubstituted may be converted into a corresponding compound wherein R2 is substituted, typically by standard alkylation procedures, for example by treatment with a haloalkyl derivative in the presence of sodium hydride and N,N-dimethylformamide, or with a hydroxyalkyl derivative in the presence of triphenylphosphine and diethyl azodicarboxylate. Furthermore, a compound of formula I initially obtained wherein R2 represents cyano(C1-6)alkyl may be converted into the corresponding 3-substituted 1,2,4-triazol-5-yl(C1-6)alkyl analogue by treatment with the appropriate acyl hydrazine derivative in the presence of a base such as sodium methoxide. Similarly, a compound of formula I initially obtained wherein R2 represents an optionally substituted propargyl moiety may be converted into the corresponding 1,2,3-triazolylmethyl analogue by treatment with azide anion. A compound of formula I initially obtained wherein the R2 substituent is substituted by a halogen atom, e.g. chloro, may be converted into the corresponding compound wherein the R2 substituent is substituted by a di(C1-6)alkylamino moiety by treatment with the appropriate di(C1-6)alkylamine, typically with heating in a solvent such as 1,4-dioxane in a sealed tube.
Where the above-described processes for the preparation of the compounds according to the invention give rise to mixtures of stereoisomers, these isomers may be separated by conventional techniques such as preparative chromatography. The novel compounds may be prepared in racemic form, or individual enantiomers may be prepared either by enantiospecific synthesis or by resolution. The novel compounds may, for example, be resolved into their component enantiomers by standard techniques such as preparative HPLC, or the formation of diastereomeric pairs by salt formation with an optically active acid, such as (xe2x88x92)-di-p-toluoyl-d-tartaric acid and/or (+)-di-p-toluoyl-1-tartaric acid, followed by fractional crystallization and regeneration of the free base. The novel compounds may also be resolved by formation of diastereomeric esters or amides, followed by chromatographic separation and removal of the chiral auxiliary.
During any of the above synthetic sequences it may be necessary and/or desirable to protect sensitive or reactive groups on any of the molecules concerned. This may be achieved by means of conventional protecting groups, such as those described in Protective Groups in Organic Chemistry, ed. J. F. W. McOmie, Plenum Press, 1973; and T. W. Greene and P. G. M. Wuts, Protective Groups in Organic Synthesis, John Wiley and Sons, 1991. The protecting groups may be removed at a convenient subsequent stage using methods known from the art.
The following Examples illustrate the preparation of compounds according to the invention.
The compounds in accordance with this invention potently inhibit the binding of [3H]-flumazenil to the benzodiazepine binding site of human GABAA receptors containing the xcex12 or xcex13 subunit stably expressed in Ltkxe2x88x92 cells.
Reagents
Phosphate buffered saline (PBS).
Assay buffer: 10 mM KH2PO4, 100 mM KCl, pH 7.4 at room temperature.
[3H]-Flumazenil (18 nM for xcex11xcex23xcex32 cells; 18 nM for xcex12xcex23xcex32 cells; 10 nM for xcex13xcex23xcex32 cells) in assay buffer.
Flunitrazepam 100 xcexcM in assay buffer.
Cells resuspended in assay buffer (1 tray to 10 ml).
Harvesting Cells
Supernatant is removed from cells. PBS (approximately 20 ml) is added. The cells are scraped and placed in a 50 ml centrifuge tube. The procedure is repeated with a further 10 ml of PBS to ensure that most of the cells are removed. The cells are pelleted by centrifuging for 20 min at 3000 rpm in a benchtop centrifuge, and then frozen if desired. The pellets are resuspended in 10 ml of buffer per tray (25 cmxc3x9725 cm) of cells.
Assay
Can be carried out in deep 96-well plates or in tubes. Each tube contains:
300 xcexcl of assay buffer.
50 xcexcl of [3H]-flumazenil (final concentration for xcex11xcex23xcex32: 1.8 nM; for xcex12xcex23xcex32: 1.8 nM; for xcex13xcex23xcex32: 1.0 nM).
50 xcexcl of buffer or solvent carrier (e.g. 10% DMSO) if compounds are dissolved in 10% DMSO (total); test compound or flunitrazepam (to determine non-specific binding), 10 xcexcM final concentration.
100 xcexcl of cells.
Assays are incubated for 1 hour at 40xc2x0 C., then filtered using either a Tomtec or Brandel cell harvester onto GF/B filters followed by 3xc3x973 ml washes with ice cold assay buffer. Filters are dried and counted by liquid scintillation counting. Expected values for total binding are 3000-4000 dpm for total counts and less than 200 dpm for non-specific binding if using liquid scintillation counting, or 1500-2000 dpm for total counts and less than 200 dpm for non-specific binding if counting with meltilex solid scintillant. Binding parameters are determined by non-linear least squares regression analysis, from which the inhibition constant Ki can be calculated for each test compound.
The compounds of the accompanying Examples were tested in the above assay, and all were found to possess a Ki value for displacement of [3H]-flumazenil from the xcex12 and/or xcex13 subunit of the human GABAA receptor of 100 nM or less.
Further, compounds of Examples 5-7 were tested in the above assay for binding to the xcex15 subunit (10 nM for xcex15xcex23xcex32 cells used as reagent giving a final concentration of 1.0 nM) and were found to possess a Ki value for the xcex15 subunit of 100 nM or less.